1. Field of the Invention
This invention relates to an improved electrostatic potential sensor using a tuning fork with associated tines. More particularly, this invention relates to an electrostatic potential sensor with improved resistance to mechanical shock and vibration and a reduced probability of operating in undesirable mechanical modes.
2. Discussion of the Related Art
Conventional electrostatic potential sensors employ a tuning fork with a pair of thin vibrating tines and a sensing electrode. The tines are configured with a relatively thin dimension and are caused to vibrate in a plane perpendicular to that dimension. The tines are interposed between the sensing electrode and the surface under test or within the electrostatic field under test.
The tines have two distinct modes of vibration. In the first mode, both tines move in unison away from and then move in unison toward a line centered between the tines. In the second mode, one tine moves toward the center line while the other tine moves away from the center line.
In the conventional electrostatic measuring apparatus, the tines on the tuning fork are caused to operate in the first mode such that the tines alternately couple and decouple the sensing electrode and the surface having an unknown quantity to be measured with the electrode. In response to the described oscillation of the tines, an AC voltage proportional to the difference between the voltage on the object under test and the voltage on the tines is induced on the sensing electrode.
The tuning fork in the conventional measuring apparatus is typically constructed with a configuration such that the tuning fork is comprised of two support legs connected to a fork root, with two tines extending from the fork root and a vane further extending from the end of each tine opposite the fork root. The vanes are typically oriented with their largest flat surfaces perpendicular to the largest flat surfaces of the tines.
The tuning fork is attached to the base of a probe by the two support legs with the tines extending parallel to the base of the probe. The electrode, which is constructed with two legs supporting a rectangular platform, is placed on the base of the probe directly under the vanes of the tuning fork with the platform being parallel to the base of the probe. Piezo electric crystals, which drive the fork, are typically mounted on the tines between the fork root and the vanes.
The connection of a conventional electrostatic measuring apparatus as an electrostatic voltmeter or field meter is shown in U.S. Pat. No. 3,921,087 issued to Vosteen. Vosteen concerns a capacitive modulator having a tuning fork with associated tines which are interposed between a sensitive electrode and a quantity being measured. A driver vibrates the tuning fork to cause the tines to oscillate in opposite phase at equal amplitude to vary the coupling relationship between the sensitive electrode and the quantity being measured. A corresponding voltage which may be processed to provide a measurement of the unknown quantity being measured is thereby induced on the sensitive electrode.
Associated circuitry comprising a preamplifier and signal amplifier are connected to the sensitive electrode to obtain desired amplification of the signal induced thereon. The tuning fork is driven by an electromechanical driver connected to a reference oscillator, and therefore, detector signals induced on the sensitive electrode as a result of coupling to the unknown quantity being measured are modulated at the frequency of the reference oscillator. Additionally, the reference oscillator supplies signals to a phase sensitive detector which also receives the output of the signal amplifier to cause demodulation of the modulated detector signals induced on the sensitive electrode. An integrating amplifier is connected to the output of the phase sensitive detector to amplify the output thereof and a voltage indicator is connected to the output of the integrating amplifier to provide measurement of the unknown quantity under test.
The conventional measuring apparatus has suffered from several problems. Being an electromechanical device, the electrostatic measuring apparatus is sensitive to mechanical shock and vibration which may damage the piezo electric crystals which serve as mechanical driving elements. In particular, when the conventional apparatus is dropped, the mechanical shock produced causes the tines to vibrate substantially beyond their normal operating amplitude, thereby cracking the piezo electric crystals and rendering the device inoperative. Further, the mechanical shock tends to bend the support legs of the fork forward such that the tines contact the electrode, thereby rendering the device inoperative.
The conventional sensor additionally suffers from the difficulty of operating improperly due to vibration occurring at the corner of the end of each tine opposite the attached vane and farthest from the fork root. These vibrations may prevent the tuning fork in the sensor from operating in the desired mode.